The covert QR codes were printed using inks that contained nanoparticles of β-NaYF4:Yb3+ , Er3+ , which up-converts into green, and NaYF4:Yb3+, Tm3+, which up-converts into blue. The final solution comprised 1 or 2 weight percent of oleic acid-capped nanoparticles along with a binding agent poly(methyl methacrylate) in toluene/methyl benzoate solvent. The patterns are easy to print on a variety of substrates and form a rigid layer that contains an even dispersion of nanoparticles.

Additional features

A variety of up-converting colours can be obtained by altering the lanthanide dopants. In their study, the scientists presented covert QR codes printed with additional security features. These extra details can be printed in many colours embedded within the QR codes, depending on the type of security needed. This approach provides another level of security as the additional features in the QR codes can be varied from micro to macro in size and excited using a single wavelength laser source.

Fabrication and testing

Full details can be found in the journal Nanotechnology.